Hitherto, as shown in FIG. 1, the kind of a frame (A) has been molded in the shape of a plate by blow molding and thereafter a reinforcement (a') is fixed thereto by means of thermal calking etc. Accordingly, addititional man hours are inevitably necessary in order to fix the reinforcement (a') to the frame (A) after the frame (A) has been molded; and moreover said frame (A) is liable to be damaged due to insufficiency of its strength while it is used for a long period of time because the frame to which the reinforcement is fixed is in the shape of a single plate.